Biochar Ovens

A number of people have requested information on
low tech biochar production. This communication concerns some biochar oven
designs that UBI/MoBI have begun working on. I have included a few pictures and
brief comments on them to give the reader some idea of the designs and their
use.

I use the term BIOCHAR OVEN
(or simply ‘oven’ in context) to designate a simple, low tech pyrolysis
apparatus dedicated primarily to the production of biochar, usually with a
feedstock capacity of 50 – 5,000 l. Also, there is no intentional admission of
oxidizers during pyrolysis into the feedstock chamber in a biochar oven.

We focus on ovens because they
are more efficient and environmentally friendly than adaptation of more
traditional methods of low tech charcoal production. The reasons for our
focusing on low tech units in this size range will be readily apparent on
reading a brief summery of the UBI concept that can be found at
http://www.biochar-international.org/regional/ubi.

‘Biochar stove’ or ‘kiln’ have
also been used for these types of units. However, biochar stoves (and stoves in
general) primary are for cooking and heating. They are usually of smaller
capacity as well. Kilns can be of very large size and often have internal
burning, particularly when used in charcoal making. ‘Retort’ is another word
that is sometimes used, but they can be of very large size and the word is not
familiar to the general public. We often deal with the general public and one
advantage I see is that ‘oven’ is readily associated by them with what the
apparatus is designed for. For all of these reasons I would encourage the use of
the term ‘biochar oven’ for these sorts of units.

The next step in developing the Hollow Core design
was to replace the simple stovepipe core with a heavy pipe of slightly larger
diameter.

2. Holes were cut in the pipe large enough to
loosely hold horizontal, open ended hollow pipes.

3. These pipes were arranged in a squed ‘X’
pattern with a hole cut in them at mid length that was faced up the core when
the oven was placed in the firing position. The loose fit of the cross pipes in
the core pipe holes allowed for p-gas (which is generated at a positive
pressure) to enter the core and burn there. Although the primary design function
of the cross pipes was to conduct heat outwards, the open ends and hole at the
middle also allowed for generated p-gas to be carried into the core for burning.
The upward rushing air in the central core may have caused a slight pressure
decrease as well as it moved around the horizontal pipe and its central, upward
facing hole. This might draw hotter gas generated nearer the core out towards
the periphery. We saw no evidence that this drew O2 into the biomass chamber
from the holes in the core pipe.

Firing was carried out in the same manner as
illustrated as for the HC ExF design (see part 2).

4. As can be seen these modifications were still
insufficient to cause good charring throughout a load of sawdust.

5. An improved heat retention cowling was made of
cemented bricks surrounded by a layer of insulating sawdust for the next trial.

6. For that trial the drum was partitioned into
vertical thirds. One each was filled with sawdust, ‘paddock’ (winter camp,
packed) manure and field dried manure. As before, the sawdust was not charred
completely through, nor was the paddock manure. However, the field dried manure
was well charred throughout.

The paddock manure had been broken up into small
pieces (though not as small as sawdust) to facilitate drying but still was not
well dried. Both factors (dampness & packing) would impede heat penetration.
However, the field dried manure was well dried and of relatively large size
making for large pore size between pieces which facilitated heat penetration.

7. A final trial was run with a drum load of
commercially available larch stove wood with a few pieces of thick larch bark.
The wood and bark were well charred. A great deal of clean burning flame exited
the top of the cowling during the burn indicating that there was insufficient O2
entering the core at the height of pyrolysis to allow for complete burning of
the p-gas there.

LESSONS & THOUGHTS

The HCF design: It seems adequate for
making biochar of dry feedstock that does not pose the heat penetration problems
of sawdust. However, a number of design innovations may make it more efficient
or practical.

‘Oil’ drums: ‘Oil’
drums (and the similar type with removable clamp lids) are made of fairly thin
gauge metal and cannot be expected to stand up to long repeated usage. However,
where available they can be inexpensive and readily adaptable and may still be
more economical or practical than similar ovens constructed from scratch using
heavier gauge metal. It would not be necessary to have a tightly fitting clamp
lid if the lid is situated at the bottom of the drum, as any escaping p-gas or
oil will be burned by the heating fire. This will be true even if the fire is
confined to the hollow core if the lid is recessed within the rim of the barrel
– the updraft in the core causes air along the bottom side of the drum to be
sucked into the core fire. This means that standard bung type drums for
containing liquids could be used by simply cutting free the lid and using the
resulting disk as the bottom, held in place during firing by removable and/or
fixed pegs.

Cylindrical polygons:
Where metal ‘oil’ drums are either not economical or unavailable, oven drums can
be fabricated from desired gauge metal. If equipment is not economically
available to fabricate round cylindrical drums and hollow cores, then
cylindrical polygons of 4 or more sides could be made by either break bending
metal sheets or welding together rectangular panels. A removable lower end need
not be epically tight fitting if it is slightly recessed for reasons described
above.

Cowling: The cowling or
oven housing is used primarily to improve heating efficiency. However the
structure can be relatively expensive and make production operations cumbersome.
With fire primarily concentrated in the hollow core, the cowling could be
reduced to insulation iatrical to the outer wall of the oven. This should
decrease initial expense while increasing mobility and ease of production
operations.

Increasing heating
efficiency: In the dry larch wood trial especially, a great deal of
smokeless flame was seen immediately above the hot gas vents of the cowling at
the height of the pyrolysis process. This indicates that the oxygen in the air
being drawn into the hollow core was insufficient to fully burn the p-gas being
vented there. It would seem that a series of channels could be designed
immediately external to the outer drum wall that could draw down this hot p-gas,
passively charge it with additional oxygen rich air and have it burn along the
outer sides of the drum. Keeping it low tech in this way would be preferable to
using some sort of forced draft which would add to operational expense and be
difficult in rural areas where electricity or fuel for auxiliary engines might
be hard to come by.

Gas tight horizontal pipes:
Drawing the hot p-gas being generated nearer the core into the outward lying
biomass may enhance heat penetration efficiency. This might be facilitated by
sealing the connection between the horizontal pipe and the core. This could be
done by either mating them with a screw fitting or welding them together. If the
latter were done with the originally designed through-pipe with upward facing
exit port, advantage could be taken of the greater pressure differential. A more
even distribution of heat through the biomass load might be achieved by
connecting each horizontal cross pipe to a feeding ring around the inside of the
wall of the drum with inlet ports evenly distributed around the circumference.

Up-sizing: Fabrication
of the drums would free us to increase drum volume through our size range of
interest (50 – 5,000 l). Merely making the drum longer would probably have
little effect on the other details of design except for accommodating the
oxidation of the greater volume of p-gas generated. This might require
increasing the diameter of the core and capacity of the outer drawdown channels
if they are used. However, increasing the diameter of the drum would also
require adjusting the diameter of the hollow core for adequate heat penetration
of the biomass. Perhaps it would become necessary to switch to a pattern of
multiple hollow cores within the drum as the diameter increases.

Horizontal burning: We
have begun testing this design with the drum stood vertically to accommodate the
external fire of the initial design phase and for the expediency of simplifying
the makeshift cowling design. With fire confined primarily to the hollow core
(and ‘draw down’ channels if they are incorporated) and a properly designed
cowling, it should be able to operate the oven in a horizontal position which
might facilitate biochar production operations, especially with longer drums.

Adaptation for sawdust:
Although the current design was inadequate to completely char a load of sawdust,
the design should be adaptable to sawdust by adjusting the diameter of the
hollow core to the degree of heat penetration achievable by the burning of p-gas
generated and any auxiliary fuel that is economically available, as indicated by
the ‘20 cm Box’ design. (Details of the 20cmB design to follow.) However, this
would decrease the volume of the load, though perhaps a second or third
concentric drum could be added with the firing occurring between them.

Analysis of performance:
In addition to testing some or all or the modifications alluded to above we plan
to characterize the performance of these designs, including temperature profiles
in various regions of the drums during operation using thermisters and data
loggers as well as characterizing biochars produced from various feedstocks.

Open source designs: To
the extent that these designs are not covered by priority or patents unbeknownst
to me (I consider the Hollow Core designs original work), I consider the designs
“open source” and invite their use and improvement asking only that I be kept in
the loop. I hope to continue to work on improving the design and to make
available the results, including detailed data, on the UBI and other web sites.
I would like to invite all interested to collaborate in creating a data pool on
the Hollow Core design, its construction, operation, maintenance and the
characteristics of the biochars produced. You can contact me at
pattamo_kop@yahoo.com.

I would like to thank the MoBI
NGOs Mongolian Women Farmer’s Association for making their facilities and labor
available for this work and People Centered Conservation of Mongolia for helping
with funding.